Isarithmic Mapping of Precipitation Data in Washington

 

This week, the cartography lab focused on creating an isarithmic map of average annual precipitation in Washington State using a variety of raster symbology techniques. The data came from the PRISM Climate Group and was downloaded from the USDA Geospatial Gateway. There are many methods for interpolating climate data from monitoring stations to grid points. While some perform well in flat terrain, few effectively capture the complex climatic variations found in mountainous areas. PRISM addresses this challenge by incorporating a conceptual framework that accounts for orographic effects such as elevation, aspect, and slope. Precipitation data collected between 1981 and 2010 in Washington was interpolated using the PRISM Model (Parameter-elevation Regressions on Independent Slopes Model), which blends monitoring station data with a digital elevation model (DEM) to generate a climatological average over the 30-year period, to account for topographic variations that occur in mountainous regions.

Figure 1

Continuous Tones

Continuous tone symbology is a method of displaying raster data where values are represented with a smooth gradient of colors, rather than distinct classes or categories. This symbology method is useful for showing continuous data, like precipitation, where values change gradually across space. The initial raster, “precipann_r_wa,” was brought into ArcGIS Pro and symbolized using a continuous tone color ramp specific to precipitation. To enhance the surface visualization, I added a ‘hillshade effect’, which uses elevation to visually emphasize terrain, in ArcGIS Pro via the hillshade function. The result was a smooth and natural-looking precipitation surface.

Hypsometric Tints

To move from continuous to discrete symbology, I used the Int tool to convert the raster’s values into integers. This allowed me to classify the data into 10 equal intervals using the Classify option in the Symbology pane. Each range is a distinct color from the precipitation ramp to create hypsometric tints, a method that divides elevation or continuous data (in this case, precipitation) into visually distinct color bands. The combination of the precipitation symbology and the legend makes the spatial distribution of rainfall easier to interpret at a glance.

Overlay Contours

As hypsometric tins cannot be displayed without contour lines, I then added overlaying contour lines using the Contour List Spatial Analyst tool. Using the “Annual Precipitation (in)” raster dataset for the input layer, I manually added contour elevations in the Contour Value Fields that matched the 10 classes displayed in the hypsometric tint symbology classes creating a “Contours” output dataset. Contours help reinforce the boundaries between precipitation zones and add another visual cue for interpreting variation in the data. With both contours and tints overlaid, the map clearly shows the relationship between precipitation patterns and elevation, especially in areas with steep terrain.

Summary

This lab was a fun exercise in translating complex, continuous climate data into a readable and visually engaging map. By walking through different symbolization techniques (continuous tone, hypsometric tinting, and contours), I gained a better understanding of how to tailor symbology to different data types and mapping goals.